As Britain and the rest of the world remains on high alert for possible terrorist attacks, governments have been forced to consider scenarios that once seemed highly unlikely. Gerry Northam investigates one of our deepest fears: that a terrorist group could instigate some form of nuclear attack, with thousands perhaps even millions of civilian causalities.

Since the early 90s, there have been hundreds of smuggling incidents involving the illicit trafficking of nuclear contraband. But could a terrorist group have acquired enough weapons-grade material to construct their own nuclear device? Would they have been able to steal a weapon from the stockpiles of the former Soviet Union? What could happen if a radiological dirty bomb went off in a city centre? Or, could a determined group repeat the success of the World Trade Centre attacks but this time, fly a plane into a nuclear facility?

The imagined terrors are almost limitless but which of these scenarios represent a real threat? And is enough being done to prevent the unthinkable?